Geology, hydrothermal alteration and gold mineralization at the Ormaque Deposit, Abitibi subprovince, Val-d'Or, Quebec

Abstract

The Ormaque deposit is located within the renowned Val-d’Or mining camp of the Archean Abitibi greenstone belt, Quebec, where the fault valve model for the formation of orogenic gold deposits originated. Ormaque is hosted by a syn-volcanic intrusion, the C-porphyry, which was emplaced in intermediate to felsic volcaniclastic rocks and flows of the ca. 2704 Ma Val-d’Or Formation. The deposit consists of shallowly-dipping quartz-tourmaline-carbonate veins associated with steeply north-dipping and east-striking reverse faults, containing few fault-fill veins of similar composition. It differs from other deposits in the camp by the marked predominance of extension veins over fault-fill veins. Reverse faults at Ormaque are severely misoriented relative to the bulk tectonic shortening direction and can only be reactivated at very low differential stresses and high fluid pressures. This favors the formation of extensional veins over a wider range of differential stresses, explaining why Ormaque differs from other deposits in the camp.